This invention relates to the separation of carbon dioxide from gas mixtures. More specifically, the invention relates to a method for the separation of carbon dioxide from a gas mixture using an Immobilized Liquid Membrane that contains a dendrimer selective for carbon dioxide.
Gas separation using facilitated transport membranes (FTMs) has been the subject of considerable research for many years. Major advantages of FTMs over conventional polymeric membranes include higher fluxes for reacting gas species like carbon dioxide, olefins and the resultant high selectivities over nonreacting species like nitrogen, paraffins etc. This is possible due to the additional mechanism of a reversible chemical reaction of the preferred gaseous species with a reactive carrier present in the FTM in addition to the solution-diffusion mechanism. FTMs are particularly attractive at low reacting species concentrations where the concentration driving force for the solution-diffusion membranes is very low (Meldon, J. H.; Stroeve, P.; Gregoire C. E. Facilitated Transport of Carbon Dioxide: A Review. Chem. Eng. Commun. 1982, 16, 263-300; Ho, W. S.; Dalrymple, D. C. Facilitated Transport of Olefins in Ag+-containing Polymer Membranes. J. Membr. Sci. 1994, 91, 13-25). Facilitated transport membranes include ion-exchange membranes, fixed-site carrier membranes, contained liquid membranes, and immobilized liquid membranes (Way, J. D.; Noble R. D. Facilitated Transport. In Membrane Handbook; Ho W. S. W.; Sirkar K. K. (Eds.) Chapman and Hall, New York, 1992).
Immobilized liquid membranes (ILMs) contain a liquid solution immobilized in the pores of the polymeric or ceramic substrate by physical forces. They are also referred to as supported liquid membranes (SLMs), particularly in the context when feed and sweep sides are liquid streams. The liquid solution consists of a carrier and a solvent. The carrier reacts reversibly with the gas species of interest.
ILMs can potentially provide the highest fluxes and selectivities for reacting species such as carbon dioxide and olefins particularly at low concentrations in gas separation. Despite the obvious advantages offered by the immobilized liquid membranes, commercialization of these membranes has not taken place due to the inherent limitation of stability of the liquid membranes. The main reasons for the instability of the ILMs are due to absence of any chemical bonding of the carrier to the substrate matrix; evaporation of the carrier species and/or the solvent liquid into the gas phases during the operation; and lower breakthrough pressures associated with the liquids.
There are variations to using these liquids for CO2 separation. The CO2 absorption can be performed in one membrane module (with CO2-containing gas flowing on one side, the absorbing liquid flowing on the other side of the membrane), while the absorbing liquid is regenerated in a separate unit called a stripping unit (it can be a membrane-based or a non-membrane based unit). The absorbing liquid can contain a facilitating agent or it can contain liquids having preferential solubility for CO2 over other gases, functioning as a physical solvent. This configuration is usually called absorption-stripping.
Another variation is to incorporate the facilitating agent in a polymeric network and forming a thin membrane on top of a substrate. The facilitating agent can be incorporated into the polymer network as a component of the polymer solution prior to its crosslinking (Ho and Dalrymple,1994, op. cit.; Ho, W. S. W. Membranes may be comprised of salts of amino acids incorporated into hydrophilic polymers. U.S. Pat. No. 5,611,843, Mar. 18, 1997). The facilitating agent can be incorporated into the network after forming the polymer network. (Matsuyama, H.; Teramoto, M. Facilitated Transport of Carbon Dioxide through Functional Membranes Prepared by Plasma Graft Polymerization using Amines as Carrier, in Chemical Separations with Liquid Membranes. Bartsch R. A.; Way J. D. (Eds.) ACS Symp. Series No. 642, p. 252 (1996).
The stability of aqueous-based ILMs is usually improved when the feed and sweep sides are completely humidified, minimizing the loss of solvent (water) due to evaporation (Teramoto, M.; Matsuyama, H.; Yamashiro, T.; Katayama, Y. Separation of Ethylene from Ethane by Supported Liquid Membranes Containing Silver Nitrate as a Carrier. J. Chem. Eng. Japan 1986, 19, 419-424). The long term stability of these membranes has not been established in literature and these membranes can not withstand even temporary oscillations in the humidity conditions on either side of the liquid membranes. A major factor that limited the practical applicability of such an approach is that the sweep side always requires a sweep gas, essentially diluting the permeated gases. This limitation has serious implications in downstream processing of the permeate stream or when highest possible concentrations on the permeate side are required, either for economic or environmental reasons. For example, in the separation of carbon dioxide from gas mixtures for sequestration, the permeate side should be as concentrated as technically possible in carbon dioxide to reduce the gas volumes for further transport and storage.
Another alternative way to improve the ILM stability is to use low-volatile and hygroscopic solvents like polyethylene glycol for preparation of the ILM (Meldon, J. H.; Paboojian, A.; Rajangam, G. Selective CO2 Permeation in Immobilized Liquid Membranes. AIChE Symp. Set. 1986, 248, 114; Davis, R. A.; Sandall, O. C. CO2/CH4 Separation by Facilitated Transport in Amine-polyethylene Glycol Mixtures. AIChE J. 1993, 39, 1135; Saha, S.; Chakma, A. Selective CO2 Separation from CO2/C2H6 Mixtures by Immobilized Diethanolamine/PEG Membranes. J. Membr. Sci. 1995, 98, 157). However, the performance of such a membrane has not been acceptable (Meldon et al., 1986, op. cit.).
It is towards the use of dendrimer-containing immobilized liquid membranes, carriers and solvents therefor in the separation of carbon dioxide from gas mixtures, that the present invention is directed.
The citation of any reference herein should not be deemed as an admission that such reference is available as prior art to the instant invention.
In accordance with the present invention, a method for the separation of carbon dioxide is described that uses an immobilized liquid membrane containing a dendrimer and, optionally, at least one solvent having carbon dioxide selectivity, such as, but not limited to, glycerol, polyethylene glycol, water, refrigerated methanol, NMP, or glycerol carbonate. Porous ceramic membranes may also be used. Other solvents may be used. In another embodiment, the method involves using a dendrimer selective for carbon dioxide and capable of forming a film as the membrane itself, optionally with at least one solvent.
A preferred dendrimer selective for carbon dioxide is a polyamidoamine dendrimer (PAMAM), but the invention is not so limiting and other dendrimers with carbon dioxide selective properties may be used, such as those with multiple terminal amino groups and those also with amido groups, secondary or tertiary amines, or combinations thereof. In a more preferred embodiment, a generation zero polyamidoamine dendrimer is used. In most preferred embodiments, the aforementioned dendrimer is used with a glycerol solvent or with glycerol carbonate, which acts both as a solvent and as a further selective carbon dioxide carrier.
Any porous membrane may be used as the membrane portion of the immobilized liquid membrane of the invention, such as but not limited to a polypropylene membrane such as CELGARD 2500 or poly(vinylidene fluoride) membranes, and preferably, hydrophilized forms of the aforementioned exemplary membranes may be used. However, the invention is not so limiting to such membranes, and as mentioned above, a polymeric dendrimer comprising carbon dioxide selective groups, e.g. primary amino groups and secondary or tertiary amine groups, or any combination thereof may as a film-forming material itself comprise the membrane or film, optionally with at least one solvent. Other preferred porous membranes that may be used in the invention include polyacrylonitrile, regenerated cellulose, and polysulfone membranes.
The selection of porous membrane for use with the glycerol carbonate carrier is as aforedescribed. Moreover, a carbon dioxide selective dendrimer or polymer with like groups may be used to form the membrane, and glycerol carbonate used as both a carrier and solvent in the selective membrane.
The invention is directed to a method for separating carbon dioxide from a feed gas comprising exposing the feed gas to an aforementioned membrane, wherein carbon dioxide is selectively transferred across the membrane. Thus, the feed gas is desirably reduced in its concentration of carbon dioxide. Such a desirable reduction of carbon dioxide in a gas is useful in a variety of applications, including but not limited to the reuse of air for animal respiration, and improving the combustibility of flammable gases that may undesirably contain carbon dioxide. Any process where reduction or removal of carbon dioxide from a gas mixture may be desired is applicable to the invention herein.
These and other aspects of the present invention will be better appreciated by reference to the attached drawings and Detailed Description.